European Patent Publication No. 0,840,160 A2 describes a Pancharatnam-type achromatic (ie, independent of frequency/colour) reflective liquid crystal display (LCD) using a twisted nematic liquid crystal (LC) layer as part of a switchable achromatic retarder.
British Patent Application No. 9806566.7 describes an improved retarder combination for an achromatic fixed retarder and twisted nematic (TN) LC used in high resolution thin film transistor (HR-TFT) displays, which reduces threshold voltage and chromaticity and improves contrast.
S. Fujiwara et al. “Proceedings of the Fourth International Display Workshops”, Nagoya 1997, (IDW '97), p. 879 describes a reflective LCD using an achromatic fixed retarder between a linear polariser and a twisted nematic LC. This is used in the HR-TFT LCD product produced by Sharp.
Solutions for converting linear polarised light to circular polarised light by a twisted nematic layer with respect to the LC parameters retardation, twist and alignment orientation can be found in Beynon et al., Proceedings of the International Display research Conference, 1997 L34.
U.S. Pat. No. 5,361,151 (Sonehara) describes a transflective LCD comprising a TN-LC layer, an internal or external semi-reflector, and chromatic retardation plates between the LC and the front and rear linear polariser.
U.S. Pat. No. 4,093,356 (J. E. Bigelow) describes a transflective liquid crystal display capable of presenting viewable indicia to an observer positioned at the front thereof.
which is responsive to either reflection of incident ambient light entering into the display from the front thereof, or transmission of light from a source behind the display, and which utilises a reflective display of the type having a nematic liquid crystal host-guest dichroic dye cell backed by a quarterwave plate and partially reflective, partially transmissive transflector member, in conjunction with a linear polariser and a second quarterwave plate arranged between the backlighting source and the partially transmissive member.
In such a guest-host cell, the dichroic dye is regarded as a guest in the liquid crystal, because the orientation of the dichroic dye molecules simply follows that of the LC molecules. The dye molecules are generally transparent when viewed along their long axes, and opaque (ie. they absorb visible light) when viewed perpendicular to their long axes, and are hence referred to as dichroic. Consequently, by applying a voltage to the LC cell, the degree of absorption in the cell can be controlled, and the cell is therefore sometimes referred to as operating in an absorption mode.
The rear quarterwave plate is used to compensate for the front quarterwave plate so that linear polarised light impinges on the guest-host liquid crystal (GH-LC).
U.S. Pat. No. 4,315,258 (McKnight et al.) describes a visual display which has an increased readout capability due to its operation in a transflective mode. A source of ambient light and light for radiation through the display from the back together assure the increased readout capability. Previously, ambient light would degrade or wash-out the display making it nearly impossible for monitoring personnel to decipher alphanumeric or pictoral displays due to the decreased contrast. A pair of linear polarizers sandwich a twisted nematic liquid crystal and have their polarisation axes either parallel or mutually orthogonally disposed so that the crystal presents bright or dark areas in response to applied potentials. Because a partially transmitting mirror is interposed between the sandwiched liquid crystal and the radiating light source, the ambient light augments the radiated light to enhance the visual display. It should be understood that, throughout this specification, references to retardation values should be understood as effective retardation values, taking into account the twist angle of the retarder. A twisted birefringent structure (such as a TNLC) has a retardation of thickness×birefringence for a particular wavelength However, it effects a retardation which is lower or higher depending on the twist angle.